Is anything done at the kernel level portable (e.g. to Windows)? It
*can* be, in principle at least (by putting appropriate #ifdef's in
the code), but I am wondering if it is in reality.

Also, in 2005 there was an attempt to implement SSI (Single System
Image) functionality to the then-current 2.6.10 kernel. The proposal
was very detailed and covered most of the bases of task creation, PID
allocation etc across a loosely tied cluster (without using fancy
hardware such as RDMA fabric). Anybody knows if it was ever
implemented? Any pointers in this direction?